This invention relates generally to a photographic cassette camera, and more particularly to a rear wall construction for such camera.
Cameras of the class described generally have a cassette guide frame and a ground-glass frame for observation of an image, one of the frames defining an image field of the camera. The cassette type frame has planar support elements defining a support plane surrounded by lateral support elements defining lateral guide surfaces, the ground-glass frame having support runners which fit against these support elements of the cassette-type frame for mounting the ground-glass parallel to support frame at a predetermined focal distance. A rod-shaped exposure test probe, having at one end an opto-electric transducer, is mounted on a holding device on one of the frames for both sliding and pivotal movement adjacent one of the lateral support elements. Such one support element and one of the support runners lying adjacent thereto have recess therein permitting movement of the probe between a selectively adjustable measuring position within the field of a front side of the ground-glass and a rest position completely out of the image field. A photographic cassette camera having a rear wall construction of this general type is disclosed in German Pat. No. 1,216,679, and an embodiment of the aforedescribed rod-shaped exposure probe is disclosed in German Pat. No. 1,572,704.
For carrying out precise selective brightness measurements of the different photo parts within the image field, the opto-electric transducer of the light meter probe should preferably be disposed on the front side of the ground glass facing the lens of the camera, but spaced a slight distance away, while the ground glass lies as precisely as possible in the image plane of the camera.
In practice, however, it has turned out that adhering to these conditions causes difficulties in construction, whenever the camera is to be suitable for use of the so-called "international" cassettes (according to standatd sheet ASA Z38.1.5.-9151) generally customary today.
For picture sizes up to 4.times.5" (9.times.12 cm), cassettes of the type mentioned have a standard depth T=5.0.+-.0.18 mm from the support plane of the cassette to the contact plane for the reverse side of the photographic material. However, the light sensitive layer of the photographic material is on the side of the film lying away from the contact plane and therefore lies on the average, only 4.75 to 4.95 mm behind the support plane of the cassette. This distance is known as the "film focal distance." Correspondingly, the frosted front side of the ground glass is to be reset by 4.85 mm as compared to the support planes of the support runners of the ground glass frame (ground glass focal distance).
In order to be able to introduce the rod-shaped exposure test probe between the support frame of the cassette and the ground glass into a measuring position and at the same time avoid contact of the ground front side of the ground glass, which is sensitive to scratches by the probe, the probe should be at the most 4.5 mm thick, measured perpendicularly to the support plane. This is most difficult to achieve in practice because of the requirements for strength of the probe, since the probe must be at least 20 cm long and hollow for the reception of the opto-electric transducer, electric connecting lines and perhaps a mechanical linkage for shifting the transducer. In practice, the probe is at least 7 mm thick. Thus, the designer is forced to shift the ground glass farther back during the selective brightness measurements in order to provide sufficient space for the exposure test probe between the cassette-type of supporting plane and the ground glass, in order to place the exposure test probe into a measuring position on the front side of the cassette guide grame facing the lens of the camera objective. Each of the aforedescribed measurements is disadvantageous because, firstly, the ground glass is no longer in the image plane of the camera during the brightness measurements and thus the picture on the ground glass becomes blurred. Secondly, the brightness measurement may not take place directly near the image plane, and moreover, the wide-angle usefulness of the camera is impaired.